A vehicle brake control device is proposed in Japanese Patent Application Publication No. JP-A-2004-90679. The vehicle brake control device sets a target vehicle speed depending on a creep condition during creeping and changes the target vehicle speed depending on various conditions, such as vehicle running conditions, road surface conditions, and driving operations, in order to realize suitable creeping.
According to the vehicle brake control device described above, braking force must be generated for each wheel in order to set a creep speed to the target vehicle speed.
However, in cases where the vehicle is traveling on a low μ road surface (i.e., road surfaces with a low surface friction coefficient μ), for example, tendency in which driven wheels are locked will easily be greater if braking force is simply generated for each wheel.
In other words, when the braking forces are generated for each wheel, it can be considered that they may be generated by applying identical W/C pressures to respective wheels. In such case, creep torque is generated for driving wheels only, so there is a different distribution of braking force for the driving wheels and the driven wheels. For this reason, greater braking forces are distributed to the driven wheels on which creep torques are not generated compared to the driving wheels. Therefore, a balance of front and rear braking forces balance becomes unstable, and results in locking up of the driven wheels.
Such a problem becomes especially apparent during times of faster idling, such as immediately after the engine is started and while the air conditioner is running.
In addition, ABS control cannot be executed at low speeds due to the lowered accuracy of a wheel speed sensor. More suitable braking force distribution control is therefore needed in order to fully exploit a side force between the tires and the road surface when inputted braking force is transmitted to the road surface.